The present invention relates to an exhaust gas purification control apparatus of an engine and, more particularly, to an exhaust gas purification control apparatus of an engine which is provided with a NOx catalyst and executes lean fuel combustion (lean burn) operation.
Lean burn engines have been developed in order to improve a fuel consumption efficiency and attain emission control of CO.sub.2. On the other hand, it is practiced as usual means to provide a three way catalyst in an exhaust line of an engine to purify exhaust gas from the engine. However, the three way catalyst is suitable for purifying exhaust gas about a stoichiometric air fuel ratio, but it has a low NOx purification performance in a region of a lean air fuel ratio. Therefore, it is desired to develop techniques of suppressing emission of NOx into atmosphere in the lean air fuel ratio region in the case where a lean burn engine is used.
As one of the developed techniques, an exhaust gas purification control apparatus is proposed (in Japanese patent No. 2600492), which apparatus is provided with a NOx absorbent in an exhaust passage, absorbing NOx when the engine exhaust gas in a lean air fuel ratio region and releasing the absorbed NOx by lowering the oxygen concentration in the exhaust gas, wherein NOx is absorbed in the absorbent when the exhaust gas is in the lean air fuel ration region, and the absorbed NOx is released by lowering the oxygen concentration in the exhaust gas flowing in the NOx absorbent.
However, even if such an apparatus is used, there is a problem that when sulphur components are contained in the engine exhaust gas in a relation with fuel compositions, the NOx absorbent reacts with sulphur (SOx) to turn into sulphide and the NOx absorbing performance is remarkably lowered. Further, when the NOx absorbent reacts with sulphur (SOx) to turn into sulphide and the sulphide is absorbed, it is difficult to release the absorbed sulphide from the NOx absorbent and the NOx absorbent has a strong tendency to be poisoned forever, so that it is difficult to recover the NOx absorbent and the life of the NOx absorbent is shortened.
In a control apparatus of a lean burn engine with a NOx absorbent, in order to prevent the sulphide formed by reaction with sulphur from being stayed in the NOx absorbent, a technique is proposed in JP A 7-217474, in which a means for estimating a quantity of SOx absorbed in the NOx absorbent and a means for detecting the temperature of the NOx absorbent are provided in the control apparatus, and an air fuel ratio of the exhaust gas flowing in the NOx absorbent is made rich temporarily to release SOx from the NOx absorbent and recover the NOx absorbent when a quantity of SOx estimated to be absorbed in the NOx absorbent when an air fuel ratio of the exhaust gas flowing into the NOx absorbent during operation of the engine is a stoichiometric value or lean becomes over an allowable value and the temperature of the NOx absorbent is higher than a preset temperature. Further, there are proposed in JP A 6-88518, JP A 7-186785) various techniques of recovering a NOx absorbent by detecting an air fuel condition of an engine, a temperature condition of the NOx absorbent and an quantity of SOx in the NOx absorbent, and controlling the air fuel ratio of the engine to release the SOx absorbed in the NOx absorbent, as well as the above technique.
Any of the above proposed techniques is for recovering a NOx absorbent by releasing and separating SOx absorbed in the NOx absorbent from the NOx absorbent, and a means is taken for measuring and estimating a quantity, etc. of SOx contained in the NOx absorbent and controlling an engine air fuel ratio. That is, a main point of the techniques is laid on how the SOx absorbed into the NOx absorbent is excluded, and it is not a technique for preventing deterioration of the NOx absorbent by SOx, by preventing SOx from being absorbed in the NOx absorbent.
As mentioned above, once sulphur (SOx) is turned into sulphide and absorbed into the NOx absorbent, it is difficult to release the absorbed sulphide and the NOx absorbent has a strong tendency to be poisoned forever, so that it is not easy to sufficiently recover the NOx absorbent even by using the above-mentioned techniques and the problem of deterioration of the NOx absorbent is sill left.